Terminal structure modification or a technology of directly adding a functional group to a side chain or adding a functional group by grafting a polymer is used as a technology for changing the characteristics of natural rubber or a synthetic rubber (for example, see PTLs 1 to 6). Such a modified diene rubber polymer is used in, for example, a rubber composition to improve the properties thereof; and is required to improve compatibility with a filler such as silica.
To improve dispersibility of silica in a diene rubber polymer, a technology of producing a silica masterbatch by mixing a silica slurry with a polymer emulsion such as a natural rubber latex, followed by coagulation and drying, is known (for example, see PTLs 7 and 8). Such a silica masterbatch is obtained by mixing silica with a rubber polymer in water (that is, wet mixing), and is called a wet masterbatch. However, a material having a hydrophilic silanol group on the surface thereof such as silica, frequently involves the case that incorporation of the material in a hydrophobic rubber polymer becomes insufficient. Therefore, there is a technology of improving the incorporation property by hydrophobicizing the surface of silica. However, dispersibility of hydrophobicized silica in water is deteriorated in wet mixing. As a result, the silica incorporated is likely to be present in a rubber polymer in a coagulated state, and this may deteriorate uniformity in kneading a rubber composition.
PTL 9 listed below discloses a depolymerized natural rubber useful as an adhesive, a pressure-sensitive adhesive or the like. In this literature, a liquid depolymerized natural rubber having a number average molecular weigh of from 2,000 to 50,000 is produced by subjecting a deproteinized natural rubber dissolved in an organic solvent to air oxidation in the presence of a metal catalyst to depolymerize the deproteinized natural rubber. This literature discloses that a main chain is decomposed by air oxidation to form a molecular chain having a carbonyl group at one terminal and a formyl group at other terminal, and the formyl group is recombined by aldol condensation. However, in this literature, the depolymerization is conducted in a solution of an organic solvent, and this literature does not disclose that the recombination is performed by changing a system containing a decomposed polymer to basicity from acidity or to acidity from basicity. Furthermore, this literature has an object to obtain a liquid depolymerized natural rubber by decomposing a natural rubber into low molecules, and does not suggest a composite with silica as a reinforcing agent.